The Sunni-Shia split has its origins in a dispute over the succession to Muhammad.

Analysis: The BBC's Paul Adams in Baghdad

It's easy to dismiss the latest crop of Isis videos and statements as mere propaganda (however well produced), but the announcement of the establishment of a caliphate is rich with religious, cultural and historic significance.

Generations of Sunni radicals have dreamt of a moment when, in the words of Isis spokesman Abu Muhammad al-Adnani, Muslims "shake off the dust of humiliation and disgrace" and a new caliphate rises out of the chaos, confusion and despair of the modern Middle East.

Many Sunnis, and all Shia, will recoil in horror from the barbarism that has accompanied this moment, but the sight of old colonial-era boundaries being erased is a powerful statement, designed to attract new recruits to this whirlwind jihad.

In one of the Isis videos uploaded on Sunday, a bearded fighter called Abu Safiyya guides the viewer around a newly demolished border post. The video, with its arresting imagery and impressive production values, is designed to electrify the group's followers.

The fact that Abu Safiyya is described as being from Chile merely adds to what the authors hope is now the organisation's global appeal.

Isis said the Islamic state would extend from Aleppo in northern Syria to Diyala province in eastern Iraq.

Abu Bakr al-Baghdadi, the group said, would become the leader of the state and would be known as "Caliph Ibrahim".

In the recording, the rebels also demanded that all Muslims "pledge allegiance" to the new ruler and "reject democracy and other garbage from the West".

What is a caliphate?

An Islamic state ruled by a single political and religious leader, or Caliph

Caliphs are regarded by their followers as successors to the Prophet Muhammad and the leader of all Muslims

First caliphate came into being after Muhammad's death in 632

In the centuries which followed, caliphates had dominion in the Middle East and North Africa

The last widely accepted caliphate was abolished in 1924 by Turkish leader Kemal Ataturk after the collapse of the Ottoman Empire

The Ahmadiyya sect of Islam has recognised a caliphate for the last century, but it is only this group that does so

On Sunday, Iraqi government jets struck at rebel positions and clashes broke out in various parts of Tikrit, witnesses and officials said.